Automobile engines operate at light loads most of the time. It is, therefore, imperative to achieve high fuel efficiency of the engine at light loads if good average fuel economy of the automobile is to be assured. Unfortunately, the light load fuel efficiency of a spark ignition type engine is low. One reason for this is due to high pumping losses that occur in an engine in which the air flow is controlled by throttling. Another reason is that the engine size is usually larger than needed for light load operation in order to assure adequate engine torque for good vehicle accelerations. A further reason is that engine knock considerations force the engine designer to set the compression ratio at a relatively low value. This adversely affects the cycle efficiency since efficiency is largely a function of engine compression ratio, which is approximately equal to the expansion ratio in a conventional engine. A still further reason is that at light load, friction consumes a much larger fraction of the indicated work than is the case at heavy load, and even more so due to the fact that the engine is much larger than needed for light load operation.
An engine that minimizes the above disadvantages is a variable cycle engine running on the Otto engine timing cycle at full load, when the compression ratio is approximately equal to the expansion ratio, and on the Atkinson cycle during partial loads, when the expansion ratio and, therefore, the engine efficiency, is increasing with a decrease in the load.
The latter engine operates on a fixed valve event timing schedule at full load for power; and operates at part loads with phaseshifting of the intake and exhaust events and varying the combustion chamber clearance volume to maintain a desired effective compression ratio. Phaseshifting the intake and exhaust events also varies the amount of residual gas left in the cylinder and the intake charge volume that is trapped without throttling of the air flow, thereby eliminating pumping losses.
An example of an engine operating under the Otto and Atkinson cycles in which the compression ratio is maintained effectively constant while the expansion ratio increases is illustrated in FIG. 1A. This figure shows some of the engine operating parameters as functions of the engine load expressed as the indicated mean effective pressure (IMEP). The figure shows that during part-load operation, both the volume of intake charge trapped in the cylinder and the clearance volume between the piston and the top of the combustion chamber vary with engine load so that the ratio of the two variable volumes remains constant. This is illustrated in FIG. 1B which also shows that the expansion ratio increases with decrease in IMEP.
Since efficiency varies with the change in expansion ratio, an increase in expansion ratio increases the engine efficiency during part loads and, therefore, is very desirable. Such a mode of operation as described can achieve significant improvement in part-load fuel efficiency in comparison to what can be achieved with conventional modes or methods of engine control. For a spark ignition variable cycle engine, the improvement is due to two factors: eliminating the pumping loop work associated with throttling, and the improved thermodynamic cycle efficiency associated with the increased expansion ratio, as mentioned above.
FIGS. 2A and 2B illustrate the above concept, showing ideal pressure-volume (P-V) diagrams for full-load and part-load operations at equal air-to-fuel ratios. At full-load, a trapped volume of intake charge is equal to cylinder volume V.sub.1a and the cylinder clearance volume is V.sub.2a. The expansion ratio is approximately equal to the compression ratio. This type of cycle is referred to as the "Otto cycle."
FIG. 2B shows diagramatically the operation under the Atkinson cycle. At part-load, the beginning of compression is delayed, and the trapped volume is reduced to V'.sub.1b. At the same time, the clearance volume is reduced to V.sub.2b, so that the compression ratio remains essentially equal to what it was at the full load Otto cycle operation. The expansion ratio, however, is much larger than it was at the full load of FIG. 2A. It will be seen that the smaller the engine load, the larger the increase in expansion ratio; that is, the smaller the charge volume, the smaller the clearance volume to maintain a constant compression ratio; therefore, the larger the expansion ratio.
It should be noted that without the ability to vary the clearance volume, the delay (or advance) of the intake valve closing to control the charge volume could not be carried out to an extent that would permit complete elimination of throttling at light load. This is because of reduction in the effective compression ratio normally associated with late (or early) intake valve closing. If carried too far, the detrimental effect of reduced effective compression ratio would exceed the benefits of reduced throttling. For example, the compression end temperature would be too low, and the burn rate too slow, resulting in unstable combustion.
In addition to reducing the volume of the trapped intake charge by late or early intake valve closing, the quantity of air to be taken in can also be varied by changing the amount of residual gas in the cylinder. This can be accomplished by varying the timing of the exhaust valve closure. The later the exhaust valve closes, the larger the quantity of residual gas retained in the cylinder, up to a predetermined maximum. Not only does the residual gas reduce the air content of the charge, but it increases its mean temperature, further reducing the mass of the trapped charge. The late exhaust valve closing and the associated increased amount of residuals also contributes to lower nitrogen oxide emissions because the peak combustion temperatures are lower and can eliminate the need for an external exhaust gas recirculating system.
The invention is directed to such a variable cycle engine as described, and more particularly to a method of phaseshifting an engine having multiple intake valves per cylinder for better air flow control.